My Thetford is set up similarly.
It relies on the rubber gasket on the gate valve in the bottom of the bowl for the seal, along with a small amount of water, left in the bowl.
After flushing, close the valve but leave some water in the bottom,
then check back in awhile and make sure the water is still there.
If not, then that rubber seal is likely in need of replacing.
O LLOYD
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